gistfile1.clj

(def queue  (ref (with-meta
                   clojure.lang.PersistentQueue/EMPTY
                   {:tally 0})))
(def seats  3)

(defn debug
  [msg n]
  (println msg (apply str (repeat (- 35 (count msg)) \space)) n)
  (flush))

(defn the-shop
  [a]
  (debug "[c] entering shop" a)
  (dosync
   (if (< (count @queue) seats)
     (alter queue conj a)     
     (debug "[s] turning away customer" a))))

(defn the-barber
  [st q]
  (Thread/sleep (+ 100 (rand-int 300)))
  (dosync
   (when (peek @q)
     (debug "[b] cutting hair of customer" (peek @q))
     (ref-set queue (with-meta (pop @q)
                      {:tally (inc (:tally (meta @q)))})))))

(add-watcher queue :send (agent 'barber) the-barber)

(doseq [customer (range 1 20)]
  (Thread/sleep (+ 100 (rand-int 200)))
  (send-off (agent customer) the-shop))

(Thread/sleep 2000)
(println "[!] " (:tally (meta @queue)) "customers got haircuts today")


=====>

[c] entering shop                    1
[c] entering shop                    2
[c] entering shop                    3
[b] cutting hair of customer         1
[b] cutting hair of customer         2
[c] entering shop                    4
[c] entering shop                    5
[b] cutting hair of customer         3
[c] entering shop                    6
[b] cutting hair of customer         4
[c] entering shop                    7
[c] entering shop                    8
[s] turning away customer            8
[b] cutting hair of customer         5
[b] cutting hair of customer         6
[c] entering shop                    9
[c] entering shop                    10
[c] entering shop                    11
[s] turning away customer            11
[b] cutting hair of customer         7
[c] entering shop                    12
[b] cutting hair of customer         9
[c] entering shop                    13
[c] entering shop                    14
[s] turning away customer            14
[b] cutting hair of customer         10
[b] cutting hair of customer         12
[c] entering shop                    15
[b] cutting hair of customer         13
[c] entering shop                    16
[b] cutting hair of customer         15
[c] entering shop                    17
[b] cutting hair of customer         16
[c] entering shop                    18
[b] cutting hair of customer         17
[c] entering shop                    19
[b] cutting hair of customer         18
[b] cutting hair of customer         19
[!]  16 customers got haircuts today